bslmf_unwrapreference.h

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/// @file bslmf_unwrapreference.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmf_unwrapreference.h                                            -*-C++-*-
#ifndef INCLUDED_BSLMF_UNWRAPREFERENCE
#define INCLUDED_BSLMF_UNWRAPREFERENCE
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmf_unwrapreference bslmf_unwrapreference
/// @brief  Provide a meta-function to unwrap reference wrappers.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmf
/// @{
/// @addtogroup bslmf_unwrapreference
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmf_unwrapreference-purpose"> Purpose</a>
/// * <a href="#bslmf_unwrapreference-classes"> Classes </a>
/// * <a href="#bslmf_unwrapreference-canonical-header"> Canonical Header </a>
/// * <a href="#bslmf_unwrapreference-description"> Description </a>
///   * <a href="#bslmf_unwrapreference-usage"> Usage </a>
///     * <a href="#bslmf_unwrapreference-example-1-unwrap-reference-wrapped-types"> Example 1: Unwrap Reference Wrapped Types </a>
///
/// # Purpose {#bslmf_unwrapreference-purpose}
/// Provide a meta-function to unwrap reference wrappers.
///
/// # Classes {#bslmf_unwrapreference-classes}
///
/// -  bsl::unwrap_reference: standard meta-function to unwrap reference wrappers
/// -  bsl::unwrap_reference_t: alias to the return type of the meta-function
///
/// # Canonical Header {#bslmf_unwrapreference-canonical-header}
/// bsl_functional.h, bsl_type_traits.h
///
/// @see  bslmf_referencewrapper, bslmf_unwrap_ref_decay
///
/// # Description {#bslmf_unwrapreference-description}
/// This component defines a meta-function `bsl::unwrap_reference`
/// that may be used to unwrap a `bsl::reference_wrapper` (which is an alias of
/// `std::reference_wrapper` when that exists) of some type `U`, resulting in
/// `U&`.  In case the specified type template argument is not a specialization
/// of `bsl::reference_wrapper` the result is the type itself.
///
/// `bsl::unwrap_reference` meets the requirements of the @ref unwrap_reference 
/// template defined in the C++20 standard [meta.trans.other].
///
/// ## Usage {#bslmf_unwrapreference-usage}
///
///
/// In this section we show intended use of this component.
///
/// ### Example 1: Unwrap Reference Wrapped Types {#bslmf_unwrapreference-example-1-unwrap-reference-wrapped-types}
///
///
/// Suppose that we work in a programming environment where types may be
/// presented wrapped in a `bsl::reference_wrapper` or its `std` equivalent.  We
/// would like to use a reference to the wrapped type, but use unwrapped types
/// as they are.  This is the exact use case for `bsl::unwrap_reference`.
///
/// First, we create types that represent both reference-wrapped, and normal
/// type parameters:
/// @code
/// typedef bsl::reference_wrapper<int *>  WrappedType;
/// typedef int                           *NotWrappedType;
/// @endcode
/// Next, we create types that are references if they were wrapped:
/// @code
/// typedef bsl::unwrap_reference<WrappedType>::type    UnwrappedWrapped;
/// typedef bsl::unwrap_reference<NotWrappedType>::type UnwrappedNotWrapped;
/// @endcode
/// Finally we can verify that the wrapped type became a reference, while the
/// other type is unchanged:
/// @code
/// assert((true == bsl::is_same<UnwrappedWrapped,    int *&>::value));
/// assert((true == bsl::is_same<UnwrappedNotWrapped, int *>::value));
/// @endcode
/// Note, that (when available) the `bsl::unwrap_reference_t` avoids the
/// `::type` suffix and `typename` prefix when we want to use the result of the
/// `bsl::unwrap_reference` meta-function in templates.
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmf
 * @{
 */
/** @addtogroup bslmf_unwrapreference
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bslmf_referencewrapper.h>
 
#include <bsls_compilerfeatures.h>
#include <bsls_libraryfeatures.h>
 
#if BSLS_LIBRARYFEATURES_HAS_CPP20_BASELINE_LIBRARY
  #include <type_traits>  // 'std::unwrap_reference', 'std::unwrap_reference_t'
#endif
 
                        // =======================
                        // struct unwrap_reference
                        // =======================
 
namespace bsl {
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP20_BASELINE_LIBRARY
using std::unwrap_reference;
using std::unwrap_reference_t;
#else
 
/// This `struct` template implements the @ref unwrap_reference  meta-function
/// defined in the C++20 standard [meta.trans.other], providing an alias,
/// `type`, that returns the result.  `type` denotes the same type as the
/// (template parameter) `t_TYPE` unless `t_TYPE` is a specialization of
/// `bsl::reference_wrapper` (which is an alias of `std::reference_wrapper<U>`
/// if that exists) for a type `U`, in which case `type` is `U&`.
///
/// See @ref bslmf_unwrapreference 
template <class t_TYPE>
struct unwrap_reference {
    // TYPES
 
    /// This `typedef` is an alias to the template parameter `t_TYPE`.
    typedef t_TYPE type;
};
 
/// This partial specialization is chosen when the template parameter `t_TYPE`
/// is a `bsl::reference_wrapper`.
template <class t_WRAPPED_TYPE>
struct unwrap_reference<bsl::reference_wrapper<t_WRAPPED_TYPE> > {
    // TYPES
 
    /// This `typedef` is an alias to the template parameter `t_WRAPPED_TYPE&`.
    typedef t_WRAPPED_TYPE& type;
};
 
#if defined(BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES)
// ALIASES
 
/// @ref unwrap_reference_t  is an alias to the return type of the
/// `bsl::unwrap_reference` meta-function.
template <class t_TYPE>
using unwrap_reference_t = typename unwrap_reference<t_TYPE>::type;
#endif  // BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
 
#endif  // else of BSLS_LIBRARYFEATURES_HAS_CPP20_BASELINE_LIBRARY
 
}  // close namespace bsl
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2024 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------
 
/** @} */
/** @} */
/** @} */